Validation of the reciprocity law for coating photodegradation

Abstract

Accelerating the photodegradation of commercial polymeric materials has great practical importance in the weathering community. However, questions exist as to whether high radiant flux exposure results can be extrapolated to in-service exposure levels. Based on the reciprocity law, the photoresponse of a material is dependent only on the total energy to which the specimen is exposed, and is independent of the exposure time and the intensity of the radiation taken separately. An experiment to validate the applicability of the reciprocity law for polymeric coatings has been carried out using the NIST integrating sphere-based ultraviolet (UV) weathering device. A nonpigmented, non-UV stabilized acrylicmelamine coating was exposed to six different UV radiation intensities ranging from 36 W/m 2 to 322 W/m 2 , and in the spectral region between 290 nm and 400 nm. Chemical changes in the coating due to UV exposure were measured with transmission Fourier transform infrared (FTIR) spectroscopy and UV-visible spectroscopy. Using two dose-damage models, the reciprocity law photoresponse for this polymeric system was verified for different photodegradation mechanisms, including chain scission, oxidation, and mass loss. P olymeric materials used in an outdoor environment are subjected to a wide variety of stress factors, including solar ultraviolet (UV) radiation, temperature, and moisture. The physical and chemical degradation resulting from outdoor exposure is known in the materials community as weathering. For commercially viable materials, weathering is usually a slow and lengthy process, often taking five years or longer before a critical performance property is deemed to have failed. These long testing periods are costly to the manufacturer and are also a barrier to innovation and the timely introduction of new products into the market. A need exists for developing exposure strategies that can accelerate weathering and yet permit valid extrapolations or predictions of service life from accelerated levels to in-service exposure conditions. Although temperature and moisture acting alone are important stress factors in the degradation of many materials, UV radiation, in conjunction with temperature and moisture, plays a dominant role in the photodegradation of polymeric materials. It is well known that polymer photodegradation can be accelerated by increasing the radiant flux to which the material is exposed. The law of reciprocity states that the photoresponse of a material is dependent only on the total energy involved and is independent of the intensity (or irradiance) of the radiation and the exposure time taken separately. 1 Experiments in which the photoresponse of a material is measured as a function of irradiance levels are commonly called reciprocity experiments.Over the last 100 years, the use of high irradiance exposures has been successfully employed by the biological, medical, and materials weathering communities. In materials weathering, for example, high solar irradiance field exposure devices have been reco...

Predicting Coating Failure using the Central Limit Theorem and Physical Modeling

“…chemistry, then the extent of the damage can be obtained directly from Monte Carlo results or the equations above. For example, the kinetics of yellowing often appear to largely follow simple first order kinetics (8-10) as do changes in infrared absorption (12). If the curves do not follow the simple curves above, then depending on the understanding available, a more detailed Monte Carlo simulation might be useful.…”

UV Effects on Materials

“…This approach carries with it the assumption of reciprocity, that higher UV dosing over short periods will approximate lower UV levels for extended periods of time. Some sources have validated the reciprocity law for photodegradation testing (Chin et al 2005). Other sources have found that reciprocity does not apply at higher levels of irradiance (Delany and Makulec 1963).…”

“…As with microorganism inactivation, it is likely that the response is nonlinear for very low doses and for very high doses, but this does not prevent accurate testing from being conducted in the mid-ranges of UV dose. Chin et al (2005) found that an acrylicmalamine coating exposed to UV irradiance levels of 36-322 W/m 2 between 290 and 400 nm caused chemical changes in the coating, including chain scission, oxidation, and mass loss.…”

Estimating service lifetimes in weathering: an optimistic view

“…and check whether the material obeyed reciprocity. 18 Thus one can see how to use such an approach. The material variables included in the degradation rate parameter, k, also make it clear how simple acceleration factors (for calculating natural exposure lifetimes from accelerated exposures) are unlikely to apply across a broad range of materials.…”